Tokyo Electric Power Co (Tepco), which operates the tsunami-hit Fukushima Daiichi nuclear plant, is investigating the cause and could not say when the clean-up will resume, company spokesman Junichi Matsumoto said.

Fresh water is being pumped in to cool damaged reactor cores, and is becoming contaminated in the process. Around 105,000 tonnes of highly radioactive water have pooled across the plant, and could overflow within a couple of weeks if action is not taken.

In earlier tests, the water treatment system reduced caesium levels in the water to about one ten-thousandth of their original levels. The system began full operations on Friday after a series of problems involving leaks and valve flaws.

The system was suspended in early Saturday when workers detected a sharp radiation increase in the system's caesium-absorbing component, Matsumoto said. Radioactivity in one of 24 cartridges, which was expected to last for a few weeks, had already reached its limit within five hours, he said.

Japan's 11 March earthquake and tsunami knocked out power to the nuclear plant, incapacitating its crucial cooling systems and causing three reactor cores to melt. Tepco aims to bring the reactors to a stable cold shutdown state by January next year.

The water treatment system is to be eventually connected to a cooling system so the treated water can be reused. But treating the water will create an additional headache – tons of highly radioactive sludge will require a separate long-term storage space.

The Fukushima crisis shattered Japan's confidence in the safety of nuclear energy and prompted anti-nuclear sentiment. But there are also concerns that Japan will face a serious summertime power crunch unless more of its reactors get back on line.

Of Japan's 54 nuclear reactors, more than 30 – including six at Fukushima Daiichi and several others that stopped due to the quake – are out of operation.

The economy and industry minister, Banri Kaieda, said on Saturday that the rest of the nuclear plants in Japan are safe and their reactors should resume operations as soon as their ongoing regular checks are completed. He said nationwide inspections this week have found that Japanese nuclear power plants are now prepared for accidents as severe as the one that crippled Fukushima Daiichi.

Resumption of about a dozen reactors undergoing regular checkups is up in the air amid growing local residents' fear of nuclear accidents. Many of the plants' hometown officials have said restarting any pending reactors would be impossible amid the ongoing crisis. Kaieda, however, said Japan needs the power. "Stable electric supply is indispensable for Japan's reconstruction from the disaster and its economic recovery," he said in a statement.

The Nuclear and Industrial Safety Agency instructed Japanese nuclear operators to improve their preparedness for severe accidents earlier this month and then conducted nationwide on-site inspections.

The inspections focused on measures to reduce the risk of hydrogen explosions inside containment buildings as one of the lessons learned from the Fukushima crisis, the world's worst atomic accident since Chernobyl.

Japanese nuclear plant operators have already taken other steps to improve accident management since the disaster to maintain core cooling capacity during blackouts.

Tepco, the operator of Japan's stricken nuclear power plant, has said it is starting an operation to clean up the site's radioactive water after several glitches and delays.

Temporary storage tanks hold radioactive water at the Fukushima Daiichi nuclear plant, following the meltdown in the spring. Photograph: Ho/Reuters

Large and expanding pools of radioactive water at the Fukushima Daiichi plant, which is about 150 miles north-east of Tokyo, were in danger of spilling into the sea within a week unless action was taken, officials said. The company has pumped massive amounts of water to cool three reactors at the nuclear plant, which went into meltdown after the earthquake and tsunami on 11 March this year.

Managing the radioactive water has become a problem as the plant runs out of places to store the liquid. About 110,000 tonnes of highly radioactive water, enough to fill 40 Olympic-size swimming pools, is being held at the plant.

Tepco, the Tokyo Electric Power Company, with help from the French nuclear group Areva, the US firm Kurion and other companies, has been test-running a system decontaminating radioactive water and reusing it to cool the reactors. But in a setback that delayed the plan by about a week, water leaked on Thursday from a facility used to absorb caesium.

Junichi Matsumoto, a Tepco official, said the company was aiming to use some of the cleaned water to cool the reactors within the next few days. This would not require pumping in any fresh water.

In April the utility dumped about 10,000 tonnes of water contaminated with low-level radioactivity into the ocean, prompting criticism from China and South Korea.

Even if the water treatment is successful, Tepco next faces the task of dealing with the highly radioactive sludge that will be left over from the decontamination process. It is unclear where the sludge will be stored for the long term.

Tepco aims to complete the initial steps in limiting releases of more radiation from the plant, and in shutting down the three unstable reactors, by January next year.

Tepco said it had not made significant alterations to its timeline.

The operator said that the storage of high-radiation sludge, likely to arise from the treatment of contaminated water, and improved conditions for the plant's workers during the summer were aspects it was examining. Measures for the workers included access to more doctors, body counters that measure exposure to radiation and resting areas away from the summer heat.

The ultimate goal is to bring the reactors to a state of "cold shutdown", where the uranium at the core can no longer boil off the water that is used as the coolant. That would allow officials to move on to cleaning up the site and eventually removing the fuel, a process that could take more than 10 years.